Electrical switching devices are well known in the field of medium and high voltage switching applications. They are e.g. used for interrupting a current when an electrical fault occurs. As an example for an electrical switching device, circuit breakers have the task of opening contacts and keeping them far apart from one another in order to avoid a current flow, even in case of high electrical potential originating from the electrical fault itself. For the purposes of this disclosure the term medium voltage refers to voltages from 1 kV to 72.5 kV and the term high voltage refers to voltages higher than 72.5 kV. The electrical switching devices, like said circuit breakers, may be rated to carry high nominal currents of 4000 A to 6300 A and to switch very high short circuit currents of 40 kA to 80 kA at very high voltages of 110 kV to 1200 kV.
Because of the high nominal current, the electrical switching devices of today require many so-called nominal contact fingers for the nominal current. When disconnecting (opening) a nominal or short circuit current within the electrical switching devices, the current commutates from nominal contacts of the electrical switching device to its arcing contacts. As well, when connecting (closing) the nominal contacts of the electric switching device, the arcing contacts are connected in advance. In embodiments the arcing contacts comprise, as a first arcing contact, arcing contact fingers arranged around the longitudinal axis of the electrical switching device in a so-called arcing finger cage and, as a second arcing contact, a rod or pin which is driven into the finger cage.
During the opening process of the electrical switching device an electric arc forms between the first and the second arcing contact, an area being called arcing volume, which arc is conductive and still carries electric current even after the opening or physical separation of the arcing contacts. In order to interrupt the current, the electrical switching devices contain a dielectrically inert fluid used as a dielectric insulating medium and for quenching the electric arc as fast as possible. Quenching the electric arc means extracting as much energy as possible from it. Consequently, a part of the fluid located in the area where the electric arc is generated is considerably heated up (to around 20′000° C. to 30′000° C.) in a very short period of time. Because of its volume expansion this part of the fluid builds up a pressure and is ejected from the arcing volume. In this way the electric arc is blown off around the instant when the current is zero. The fluid flows into one or more exhaust volumes where it is cooled and redirected by a cooling device. Mixing with the cold fluid located in the exhaust volume or volumes is only possible to a relatively small extent, because the predominant part of the cold gas present inside the respective exhaust volume is pressed out of the exhaust volume by the hot fluid, which expands out of the arcing volume, before any significant mixing can occur. When the hot exhaust fluid comes into electric-field-stressed regions, e.g. close to shieldings, unwanted dielectric flashovers may occur, as the dielectric withstand capabilities of the exhaust fluid is typically lower at higher temperatures. It is therefore necessary to cool down the exhaust fluid as much as possible before it travels into such electric-field-stressed regions of the exhaust volume(s).
In EP 1 403 891 A1 of the same applicant, an SF6-gas-blast circuit breaker is disclosed in which SF6-exhaust-gas from an arcing area is passed through a hollow contact into a concentrically arranged exhaust volume, and from there into a switching chamber volume located further outward. For improved SF6-exhaust-gas cooling, at least one intermediate volume and possibly an additional volume is or are arranged concentrically between the hollow contact and the exhaust volume and are separated from one another by intermediate walls. The intermediate walls generate an increased intermediate SF6-exhaust-gas pressure and have holes or openings for forming SF6 gas jets. The SF6-exhaust-gas jets then impact on opposite walls opposing the openings and are swirled intensively at the opposing walls. Thus, the SF6-exhaust-gas is cooled by radially flowing out the SF6-switching-gas from the inner to the outer volumes through a sequence of jet-forming openings and jet-swirling opposing baffle walls, and thus a large amount of thermal energy is transferred to walls of the volumes in the exhaust system.
The openings between the hollow-contact volume, the intermediate volume and, if appropriate, the additional volume are arranged offset with respect to one another on the circumference. The openings between the additional volume and the exhaust volume are arranged offset with respect to one another on the circumference and/or in the axial direction. This also results in meandering as well as spiralling SF6-exhaust-gas paths being predetermined, with the dwell time for which the SF6-exhaust-gas remains in the exhaust area being increased, and with the heat transfer from the SF6-exhaust-gas being further improved. Furthermore, the holes can be covered by means of panels in the form of perforated metal sheets to produce a larger number of radially directed SF6-exhaust-gas streams or SF6-exhaust-gas jets. These SF6-exhaust-gas jets again strike the opposite wall, are swirled at the impact points, and thus intensively cool the hot SF6 exhaust gas. The intermediate volume, which improves the cooling, is arranged in the exhaust area on the drive contact side. A second intermediate volume may also be provided on the fixed-contact side. Overall, at least one intermediate volume is additionally required in the circuit breaker, that is to say in addition to the hollow-contact volume, the exhaust volume and the switching chamber volume, in order to achieve efficient SF6-exhaust-gas cooling.
In WO 2006/066420 of the same applicant, an SF6-gas-blast generator circuit breaker with a similar exhaust gas system is disclosed, which has intermediate walls with openings for SF6-exhaust-gas jet formation and opposing walls with baffle-wall and heat-sink function for vortex heat transfer of the SF6-exhaust-gas to such opposing walls.
In WO 2010/142346 of the same applicant, a gas-blast circuit breaker with a novel arc-exctinguishing insulation fluid comprising fluoroketones is disclosed. High voltage circuit breakers having a heating chamber for providing a self-blasting effect can be operated with such fluoroketones and specifically C6-fluoroketones. Such fluoroketones are disclosed to beneficially increase the self-blasting pressure in the heating chamber during a back-heating phase in a switching operation, as they are decomposed to a larger number of fluorocarbon compounds having a lower number of carbon atoms. Inside the arcing region, a favourable arc extinction capability of fluoroketones having from 4 to 12 carbon atoms is at least partially attributed to the recombination of the dissociation products of the fluoroketones mainly to tetrafluoromethane (CF4), which is a highly potent arc extinction medium. Moreover, C6-fluoroketones are disclosed to be useful for limiting the exhaust gas temperature in the whole vessel and in the exhaust volumes during and after arc interruption, because decomposition of sufficiently present C6-fluoroketone molecules absorbs the excess thermal energy and prevents further exhaust-gas heating beyond the decomposition temperature of around 550° C. to 570° C.
In WO 2012/080246 of the same applicant, a gas-blast circuit breaker with arc-exctinguishing insulation fluids comprising C5-fluoroketones is disclosed. The C5-fluoroketones have a non-linear increase of dielectric strength in mixtures with certain carrier gases, such as nitrogen and carbon dioxide. The C5-fluoroketones again provide a beneficial blasting-pressure increase in the compression chamber and/or heating chamber and/or arcing region during an arc-extinguishing phase due to molecular decomposition. In addition, recombination of C5-fluoroketone to tetrafluoromethane (CF4) in the arcing region is beneficial for arc extinction. As mentioned, molecular decomposition is also beneficial in the exhaust region, because the rather low dissociation temperatures of the fluoroketones of about 400° C. to about 600° C. or even 900° C. can function as a temperature barrier in the exhaust gas.
In both WO 2010/142346 and WO 2012/080246, the decomposition of fluoroketones in the heating chamber, compression or puffer chamber, arcing region and exhaust volumes are considered to be beneficial for the circuit breaker performance and in particular for the exhaust gas cooling.
In DE 10 2011 083 588 A1 an exhaust system with at least two concentric exhaust tubes is disclosed. The exhaust tubes have large numbers of radial (mantle-sided) over-pressure relief openings that are mutually off-set to one another such that direct radial gas outflow through both exhaust tubes is blocked. The relief openings may be arranged such that the exhaust gas is forced to enter the first and second exhaust tube repeatedly. Also axial (end-sided) non-overlapping over-pressure relief openings are disclosed and may e.g. be on opposite end faces of the first and second exhaust tube. An armature body can be provided, which is shiftable or dimensionally adaptable to hide or clear openings and thus to adapt the cooling capacity. Overall, exhaust gas is cooled by providing a long meandering (i.e. alternatingly radial and axial) gas path, by providing a very large number and density of openings, and also by providing each opening with an opposing baffle wall section for better mixing the exhaust gas.
In U.S. Pat. No. 7,763,821, a puffer-type gas-blast circuit breaker is disclosed which has a moveable hollow arcing contact with a radial opening for releasing exhaust gases in radial direction. The drive rod for the hollow arcing contact carries a gas blocking member for preventing axial gas discharge towards the drive unit.